'not available' error - ContentGroup is null

description

DNN 7.4.2
2SXC 8.3.4

I have a relatively simple Content module that has about 50 items. It uses a razor template. It works fine most of the time, but every once in a while, seemingly at random, the module doesn't appear and an error message appears in it's place saying "Header
Slider is not available".

I checked the event viewer and it says: PMModule Load Exception. ContentGroup entity is null.